Real time location systems (RTLS) are systems that are used to measure the location of objects in real time. RTLS have application in people, equipment, and asset tracking. A substantial number of these applications require measurement accuracy to less than a few meters.
Many existing RTLS use a measurement of the strength of a radio signal (e.g., a received signal strength indication (RSSI)) to indicate distance. RSSI-based RTLS, however, suffers from a number of problems. One problem for RSSI-based RTLS is multipath fading since multipath signals can cause dramatic changes in signal strength even over short distances. Another problem for RSSI-based RTLS is that a site survey (e.g., some position and/or radio signal environment information) is often required in order for RSSI-based RTLS to be useful. In addition, movement of people or objects within a site are also a problem for RSSI-based RTLS; the movement can cause substantial changes in the RSSI-level at a given spot. Due to these problems, RSSI-based RTLS is typically not a sufficiently precise method of measuring location.
Other existing RTLS use the propagation time of radio signals to estimate an object's location. For example, some RTLS use round-trip time of flight—either active such as two-way ranging or passive such as pulse radar, and some RTLS use time difference of arrival—such as is used in global positioning systems (GPS). One problem is that since radio signals propagate at the speed of light (approximately 1 foot in 1 ns), for timing-based RTLS to accurately measuring distance requires being able to measure the time of arrival of a radio signal to a handful of nanoseconds resolution. Another problem for some timing-based RTLS is their use of special modulated signals such as chirps or pulses to enable accurate distance measurement. For example, ultra wideband signals (UWB) use very sharp, fast (e.g., quickly oscillating) pulses to give excellent temporal resolution at a timing-based RTLS receiver; however, there are two problems with UWB systems: 1) the range of UWB signals is not particularly large due to the wide effective noise bandwidth of the receiver, and 2) there is no common frequency band approved worldwide for unlicensed UWB transmission.